Shigella species are a common cause of bacterial diarrhea worldwide, particularly in developing countries. Usually spread by contaminated water supply, these Gram-negative pathogens are capable of invading and replicating within mucosal cells of the human colon. Entry into the host cell is critical for Shigella virulence but incompletly understood. It is apparent that the bacteria manipulates host cytoskeleton and signaling pathways by means of a repertoire of proteins secreted by a type III secretion system (T3SS). As part of an effort to elucidate the bacterial entry process, my preliminary work shows that Shigella flexneri entry is decreased in the absence of host small GTPase ADP-ribosylation factor 6 (ARF6), indicating that ARF6 is important for Shigella pathogenesis. ARF6 is known to act at the host cell surface and at endosomes to regulate membrane trafficking, lipid metabolism, and actin remodeling. My preliminary studies also indicate that the T3SS effector IpgD is critical for efficient entry and ARF6 activation. Using fluorescence microscopy-based and biochemical techniques, I propose to determine how ARF6 contributes to efficient S. flexneri entry and to characterize the mechanism by which IpgD enhances activation of ARF6 during S. flexneri entry. My specific aims are to: 1. Determine the mechanism(s) by which ARF6 contributes to efficient S. flexneri entry. 2. Determine the mechanism(s) of ARF6 activation by S. flexneri. This proposal combines cell biological, biochemical, and genetic approaches to test my hypothesis and further our understanding of the mechanism(s) of Shigella entry. In addition to adding insight to current knowledge of basic cell biological processes and signaling, this work will contribute to our understanding of intracellular bacterial infection and may improve treatment of such infections, which include not only diarrheal disease caused by S. flexneri and Salmonella species but other infectious diseases such as rickettsia and tuberculosis.